Hot melt adhesives typically exist as entirely solid materials which do not contain or require any solvents. They are solid materials at ambient room temperature but can be converted to a flowable liquid or fluid state by the application of heat, in which state they may be applied to a substrate. On cooling, the adhesive regains its solid form and gains its cohesive strength. In this regard, hot melt adhesives differ from other types of adhesives, such as water-based adhesives, which achieve the solid state by evaporation, removal of solvents, polymerization, or other means.
These adhesives are particularly useful in the manufacture of a variety of industrial or consumer goods where bonding of various substrates is necessary. An advantage of hot melt adhesives is the absence of a liquid carrier, as would be the case for water-based or solvent-based adhesives, which requires a drying step during application of the adhesive. Suitable hot melt adhesives possess the appropriate bond strength to adhere the substrates involved, and also demonstrate adequate flexibility, no staining or bleedthrough of the substrate, suitable viscosity and open time to function on a variety of substrates, acceptable stability under storage conditions, and acceptable thermal stability under normal application temperature.
Hot melt adhesives may be formulated to be relatively hard and free of tack or, in contrast, to be pressure sensitive, i.e., relatively soft and tacky at room temperature. Hot melt adhesives are increasingly utilized for affixing labels to various substrates, such as to glass or plastic bottles. Pressure sensitive hot melt adhesives for labeling are usually categorized as either removable or permanent. Permanent adhesives are formulated to cause the label to tear upon removal from the substrate. On the contrary, removable adhesives must allow the label to be removed from the substrate with a clean release, i.e., leaving no residue and without the tear of the label stock which occurs in a permanent adhesive application.
Solid hot melt adhesives for permanent adhesives have been widely used for many years. However, a hot melt adhesive, and more particularly a hot melt pressure sensitive adhesive, that gives good removability has not been available. Current removal adhesives are supplied for label stock from acrylic latices and solvented solution adhesives. Both of these materials have high molecular weight polymers that reduce flow on a surface to prevent build up of adhesion. In contrast, hot melt adhesives, in particular hot melt pressure sensitive adhesives, are based on materials having lower molecular weight polymers and high amounts of very low molecular weight components that make reduced flow or wetting on a surface very difficult.
Many different polymers have been used in hot melt adhesives employed in the construction of industrial or consumer goods. Typical hot melt adhesives have employed polymers which have included tri-block copolymers such as styrene-isoprene-styrene (SIS), styrene-butadiene-styrene (SBS), styrene-ethylene-butylene-styrene (SEBS); ethylene-vinyl acetate (EVA) copolymers; and/or amorphous poly-alpha-olefin (APAO). Although these polymers, when properly blended, provide adhesion to most substrates, they are not suitable for certain particular uses. One shortcoming of the prior hot melt adhesives concerns their removability, which is an important feature for purposes of recycling the substrate.
To improve removability of the hot melt adhesive, the prior art has aimed to increase the water-solubility or water-dispersibility characteristics of the adhesive. For example, one known water sensitive hot melt adhesive composition which may be utilized in the manufacturing of disposable goods, especially disposable nonwoven articles, combines high dry bond strength with increased water solubility, thereby permitting the component elements of the disposable article to be recycled or otherwise disposed of in an environmentally friendly manner (i.e., degraded). Other known adhesives relate to a water-soluble or water-dispersible hot melt composition based on graft copolymers. These water-removable hot melt adhesives are used for labelling returnable bottles at high speeds, wherein the labels can be removed by brief soaking in hot water. However, these water-soluble compositions are not favorably employed to adhere labels to glass bottles, where water-resistant characteristics are desired as much as the clean removability of pressure sensitive labels.
Conventional styrene-isoprene-styrene (SIS) block copolymer/hydrocarbon (HC) tackifier-based hot melt adhesives have been used to adhere labels to glass bottles for decades. One of the beneficial properties of these types of adhesives for this application is that they can withstand a 7-day water immersion test. Accordingly, these types of adhesives are known for their favorable water-resistant characteristics. However, this presents a difficult challenge when the labels and adhesives need to be removed in a glass bottle recycling process. Hot caustic baths are successfully used to remove many water-based label adhesives. However, the hot melt adhesives are very resistant to caustic.